1. Field of the Invention
This invention relates generally to an AC/DC power supply device, and more specifically to an AC/DC power supply device designed to operate a refrigerator, for example, from either AC power source or DC power source wherein relatively high voltage circuits for AC operation and relatively low voltage circuits for DC operation and/or for AC operation, are adapted not to face each other in close vicinity by simple construction.
2. Description of the Prior Art
Electrical vibration type compressors are generally used in household refrigerators as well as in refrigerators on board automobiles, motor boats, etc. because they are compact in size, light in weight and have good operating efficiency. The electrical vibration type compressor usually has an inverter circuit for converting direct current into alternating current since batteries are used as a power source when it is used in a refrigerator on board an automobile, etc.
To meet such an application, an AC/DC switchable power supply device has been devised as a power supply device which can be operated on either AC power source or DC power source. An example of such a device is shown in FIG. 1.
In FIG. 1, numeral 1 refers to a transformer; 2 to a refrigerator compressor; 3 to a single-contact type thermostat for controlling temperature in the refrigerator to be cooled by the operation of the compressor 2; 4 to an inverter circuit; 5 to an input winding; 6 to an AC input winding; 7 to a winding on the inverter output terminal side; 7-1 to a center tap of the winding 7; 8 to a feedback winding; 9 to a relay for changing over the AC circuit and the DC circuit; 10, 13 and 16 to fixed contacts for DC operation in the relay 9; 11, 14 and 17 to fixed contacts for AC operation in the relay 9; 12, 15 and 18 to movable contacts in the relay 9; 19 to a relay winding in the relay 9; symbol C2 to a smoothing capacitor for smoothing voltage variations in the input fed to the inverter circuit 4, respectively.
When the refrigerator compressor 2 is operated on AC power source, current is supplied to the relay winding 19 of the relay 9 to cause the movable contacts 12, 15 and 18 to electrically connect with the fixed contact for AC operation 11, 14 and 17, respectively. Thus, a circuit from a terminal .alpha..sub.1 on the AC power source side to a terminal .alpha..sub.2 in the figure via the AC input winding 6, the fixed contact for AC operation 11, the movable contact 12, the single-contact type thermostat 3, the movable contact 15 and the fixed contact for AC operation 14 is formed, and AC power source voltage is fed to the AC input winding 6. This causes an AC low voltage of approximately 24 volts to be generated in the output winding 5 via the transformer 1 to drive the refrigerator compressor 2.
In the meantime, when an AC power source voltage is not applied to the AC input terminals .alpha..sub.1 and .alpha..sub.2, the compressor 2 is driven on DC power source. In this case, a circuit from a positive terminal of the DC power source to a negative terminal of the DC power source via the fixed contact for DC operation 13, the movable contact 15, the thermostat 3, the movable contact 12, the fixed contact for DC operation 10 and the center tap 7-1 of the inverter output terminal side winding 7 is formed because the fixed contacts for DC operation 10, 13 and 16 in the relay 9 are connected to the movable contacts 12, 15 and 18, respectively, as shown in FIG. 1, and transistors TR1 and TR2 in the inverter circuit 4 alternately repeat on-off operation to feed an AC voltage to the inverter output terminal side winding 7. Thus, an AC voltage is generated in the output winding 5 via the transformer 1 to drive the refrigerator compressor 2.
The feedback winding 8 is for controlling the switching of the transistors TR1 and TR2. The fixed contact for DC operation 16, the fixed contact for AC operation 17 and the movable contact 18 in the relay 9 are for supplying triggering current to the transistors TR1 and TR2 in the inverter circuit 4 when starting the compressor 2 on DC power source.
In the DC operation mode, the single-contact type thermostat 3 is connected to the input side of the DC circuit to prevent losses such as unwanted non-loaded running of the inverter circuit 4 under the state where the single-contact type thermostat 3 remains off, and to prevent damages of transistors, etc. caused by overvoltage generated under the above-mentioned state. The above-mentioned unwanted problems would be occurred if the single-contact type thermostat was installed in the output side.
In this type of AC/DC power supply device, however, when commercial 100-V AC power source is used as an AC power source and a 12-V battery as a DC power source, an AC voltage of approximately 100 V is applied to the fixed contacts for AC operation 11 and 14 and a DC voltage of approximately 12 V is applied to the fixed contacts for DC operation 10 and 13 in the relay 9, and the distance between the fixed contacts for AC operation 11 and 14 and the fixed contacts for DC operation 10 and 13 in the relay 9 is relatively short. Consequently, this type of AC/DC power supply device has inherent dangers of electric shock to human body or damage to other DC devices due to leakage of high voltage to the circuits for DC operation in the low voltage circuits in case withstanding voltage breakdown is caused between the fixed contacts for AC operation 11 and 14 and the fixed contacts for DC operation 10 and 13 during AC operation mode. Furthermore, in the DC operation mode, AC high voltage is induced in the AC input winding 6. And the induced AC high voltage would be leaked to the low potential points.
In addition, this type of AC/DC power supply device has the following drawbacks. As mentioned earlier, an AC power source voltage is fed to the AC input winding 6 during AC operation mode, causing an AC voltage to be generated in the output winding 5 via the transformer 1, and at the same time causing an AC voltage to be generated in the winding 7 on the inverter output terminal side via the transformer 1. This AC voltage generated in the winding 7 on the inverter output terminal side causes the smoothing capacitor C2 to be charged. In this case, the smoothing capacitor C2 repeats charging and discharging. Since the smoothing capacitor C2 serves as an apparent DC power source during discharging, and supplies discharge current to the winding 7 on the inverter output terminal side, bringing the inverter temporarily into an operating state, a voltage is induced in the output winding 5 via the transformer 1. For this reason, the voltage waveform generated in the output winding 5 does not assume a desired waveform, a sine wave, for example. In other words, an unwanted noise is superimposed on the sine wave, for example, resulting in the decreased operating efficiency of the compressor 2 and noise interference to external equipment.